1. Field of the Invention
The present invention relates to an electroluminescent display device, and more particularly, to a design for conductors in an electroluminescent display device that reduces a voltage drop in an electrode layer on the light emitting side of an electroluminescent display device having a large screen size.
2. Description of the Related Art
Since electroluminescent display devices have superior viewing angle, contrast, response time, weight, size, thickness, and power consumption compared to cathode ray tubes (CRTs) and liquid crystal displays (LCDs), much attention has been paid to electroluminescent display devices as new generation display devices. Electrolumninescent display devices can be active or passive light-emitting display devices. Electroluminescent display devices can also be classified into inorganic and organic electroluminescent display devices according to whether alight-emitting layer is made of an inorganic or organic material.
In electroluminescent display devices, a color display can be obtained in different ways, for example, by arranging individual light-emitting elements corresponding to three colors (red, green, and blue) side-by-side on a substrate, by using a blue light-emitting panel and a color conversion layer that is mounted on a front or rear substrate of the panel, or by using a white light-emitting panel and a color filter.
U.S. Pat. No. 5,059,861 to Littman et al discloses an electroluminescent display device having a cathode made of an alkaline metal and a plurality of metals other than the alkaline metal. U.S. Pat. No. 5,047,687 to VanSlyke discloses an electroluminescent display device having a cathode made of a plurality of metals, at least one of which is a low work function metal other than an alkaline metal. Here, the low work function metal is aluminum, vanadium, etc. U.S. Pat. No. 5,073,446 to Scozzafava et al, Japanese Patent Laid-Open Publication No. Hei. 5-36475, Hei. 8-222368, and Hei. 7-161474 disclose an anode, a laminated structure of a light-emitting layer, a cathode, an encapsulation layer and a sealing layer for protection of the cathode.
Meanwhile, the above-described conventional electroluminescent display devices undergo a significant voltage drop due to a long distance from a cathode power supply source to some of the pixels in a large display. For this reason, uniform luminance and definition are not ensured. The voltage drop occurs due to high resistance of the cathode electrode and lead lines. For this reason, a voltage difference between a near region and a far region from a cathode power supply source is present.
Such a voltage drop becomes more problematic in front emission type electroluminescent display devices which has an high opening ratio. U.S. Pat. No. 5,981,306 to Burrows et al. discloses an organic light-emitting device that is a front emission type electroluminescent display device. A cathode electrode layer of the organic light-emitting device in Burrows '306 is made up of a thin metal layer and an indium tin oxide (ITO) layer that is deposited on the thin metal layer. ITO is used in Burrows '306 because it is both conductive and it is transparent to light. However, use of ITO for an electrode can become problematic in electroluminescent display devices having a large screen size because ITO has a high enough resistivity to cause a voltage IR drop, especially for large screen sizes. When such a voltage drop occurs, some pixels receive less voltage than other pixels, leading to non-uniform luminance distribution across the display.
U.S. Pat. No. 5,851,709 to Grande et al. discloses an electroluminescent display device with a color filter with an organic layer. However, Grande '709 is silent regarding the electrodes used.
In case of a front emission type electroluminescent display device, it is required that an electrode adjacent to a front substrate is made of a material that is both transparent and conductive, such as indium tin oxide (ITO) and indium zinc oxide (IZO). ITO or IZO is deposited at a low temperature after forming a light-emitting layer to minimize deterioration of the light-emitting layer due to heat or plasma. However, an ITO or IZO layer thus formed at a low temperature has poor film quality and resistivity, and thus, a voltage drop phenomenon becomes significant. Consequently, there arises a problem such as non-uniform luminance distribution of images displayed in electroluminescent display devices, particularly when the screen size is very large.
Also, pixels created by light emission from a light-emitting layer undergo color interference, by which creating high-definition images is difficult. In addition, such a transparent and conductive materials like ITO generally has a high resistance, especially when there is a large screen size. Therefore, in a case where an electrode for a large-sized electroluminescent display device is made of such a material, an IR voltage drop occurs across the ITO electrode. This voltage drop on the electrode can be problematic because it causes a non-uniform luminance distribution across the large screen of the display.